Jetpack Compose has taken the Android UI development world by storm. It simplifies UI development by making it declarative and functional. But one of the first things you’ll encounter when using Jetpack Compose is the setContent function. In this blog, we’ll break down what setContent function is, how it works, and why it matters.

What is setContent Function?

In traditional Android UI development (using XML layouts), you would typically call setContentView() inside your Activity to set the screen’s layout. With Jetpack Compose, things have changed!

setContent replaces setContentView when you want to define your UI using Compose.

From now on, if you use Jetpack Compose, use this:

// This is defined for illustration purposes. You can use any composition or composable functions inside setContent{...}.

setContent {
    MyComposableContent()
}

Instead of using the older setContentView method with XML layouts.

Basic Syntax Example of setContent function

import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.material3.Text
import android.os.Bundle

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            Text("Hello, Jetpack Compose!")
        }
    }
}

How Does setContent Work?

The setContent function is part of the ComponentActivity class. It allows you to set a Composable block as your UI content.

What Happens When You Call setContent?

1. Initializes the Compose UI framework.
2. Sets the content view with a root ComposeView that renders your composables.
3. Observes recompositions to keep the UI up-to-date with state changes.

Essentially, setContent is the entry point for Jetpack Compose to build and display your UI.

Function Signature

Here’s the function definition again for reference:

fun ComponentActivity.setContent(
    parent: CompositionContext? = null,
    content: @Composable () -> Unit
)

Here,

Extension Function:
setContent() is an extension function for ComponentActivity, which means you can call it on any ComponentActivity or its subclasses like AppCompatActivity.

parent: CompositionContext? = null:

• This optional parameter represents the parent composition context. In Jetpack Compose, a CompositionContext coordinates updates and re-compositions of composable functions.
• Passing a parent helps coordinate the lifecycle of this composition with another, which is useful for nested composable hierarchies.

content: @Composable () -> Unit:

• This parameter is a composable lambda function that defines the UI contents.
• For Example,

setContent {
    Text("Hello Compose!")
}

Preview

I am going a little out of context, but it’s okay. I feel it’s important for our next discussion. Let’s think: what do you think might be inside the setContent() function body ({..})? 

fun ComponentActivity.setContent(){...}

Does Android still use the old View system at its core, or has it introduced something new? If Android has introduced a new system, how can we still support our old or legacy code written in XML?

The answer is simple: ComposeView. It acts as a bridge between Jetpack Compose and the traditional View system. This allows us to integrate Jetpack Compose with existing XML-based layouts and continue using our legacy code when needed.

Although Jetpack Compose offers a completely new way to build UIs, it does not use the old View system internally for rendering. Instead, Compose has its own rendering mechanism. However, thanks to ComposeView and interoperability APIs like AndroidView, Compose and the View system can seamlessly work together. This is why setContent() can host composables within an activity or fragment, enabling us to mix both approaches in a single app.

It’s just an additional insight. Now, let’s get back on track: inside setContent(), we’ll find a ComposeView. Let’s explore what’s inside it to better understand how it works.

public fun ComponentActivity.setContent(
    parent: CompositionContext? = null,
    content: @Composable () -> Unit
) {
    val existingComposeView = window.decorView
        .findViewById<ViewGroup>(android.R.id.content)
        .getChildAt(0) as? ComposeView

    if (existingComposeView != null) with(existingComposeView) {
        setParentCompositionContext(parent)
        setContent(content)
    } else ComposeView(this).apply {
        // Set content and parent **before** setContentView
        // to have ComposeView create the composition on attach
        setParentCompositionContext(parent)
        setContent(content)
        // Set the view tree owners before setting the content view so that the inflation process
        // and attach listeners will see them already present
        setOwners()
        setContentView(this, DefaultActivityContentLayoutParams)
    }
}

Let’s break down the body of the function:

val existingComposeView = window.decorView
    .findViewById<ViewGroup>(android.R.id.content)
    .getChildAt(0) as? ComposeView

Here, 

window.decorView:

• The root view of the window where your activity’s content resides.

.findViewById<ViewGroup>(android.R.id.content):

• android.R.id.content is the standard ID for the content view of an activity.
• This line finds the view group that holds the content of the activity.

.getChildAt(0) as? ComposeView:

• This retrieves the first child of the content view (assuming it’s a ComposeView).
• The as? ComposeView safely casts the child to a ComposeView if it’s already present.

Next, there’s a check for whether the ComposeView already exists:

if (existingComposeView != null) with(existingComposeView) {
    setParentCompositionContext(parent)
    setContent(content)
}

if (existingComposeView != null):

• Checks if a ComposeView is already present as the first child of the content view.

with(existingComposeView):

• If the ComposeView exists, this block configures it:
• setParentCompositionContext(parent): Sets the parent composition context for coordinating composition updates.
• setContent(content): Sets the new composable content to be displayed in the ComposeView.

This approach reuses the existing ComposeView if available, avoiding the need to create a new one.

If no existing ComposeView is found, a new one is created:

else ComposeView(this).apply {
    // Set content and parent **before** setContentView
    // to have ComposeView create the composition on attach
    setParentCompositionContext(parent)
    setContent(content)
    // Set the view tree owners before setting the content view so that the inflation process
    // and attach listeners will see them already present
    setOwners()
    setContentView(this, DefaultActivityContentLayoutParams)   // Note : here this is composeview as it is inside apply 
}

ComposeView(this):

• Creates a new ComposeView, passing the current ComponentActivity as the context.

setParentCompositionContext(parent):

• Sets the parent composition context for coordinating updates.

setContent(content):

• Sets the composable lambda as the content of the ComposeView.

setOwners():

• Ensures the ViewTreeLifecycleOwner and ViewTreeViewModelStoreOwner are set. These owners are necessary for handling the lifecycle and ViewModel integration properly.

setContentView(this, DefaultActivityContentLayoutParams):

• Sets the newly created ComposeView as the root view of the activity, using default layout parameters.

In short,

Reusing Existing ComposeView:

If there’s already a ComposeView, the function updates its content directly, improving efficiency by avoiding creating a new view.

Creating New ComposeView:

If no ComposeView exists, a new one is created and set as the activity’s content view.

Composition Context:

The parent parameter helps maintain the composable hierarchy and ensures updates are properly synchronized.

Lifecycle Awareness:

setOwners() ensures that the ComposeView has the necessary lifecycle owners before it gets attached to the activity.

By the way, what exactly is ComposeView?

I already gave a little hint, but there’s still more to explore. So, let’s dive into the details.

ComposeView is a special View provided by Jetpack Compose that serves as a bridge between the traditional Android View system and the Compose framework. Essentially, it allows you to embed composable UI within a standard Android View hierarchy.

Let’s break down what ComposeView is, how it works, and where it’s useful.

Overview of ComposeView

The ComposeView class extends AbstractComposeView, making it a View that can host Jetpack Compose UI components.

• Purpose: Allows seamless integration of Jetpack Compose content into existing Android View-based UI. It acts as a container for composable content in environments that primarily use Views (e.g., activities or fragments that aren’t fully migrated to Compose).
• Key Functionality: Provides a method setContent to define the Compose UI content.

Type

ComposeView does not directly extend android.view.View. Instead:

android.view.View
   └── android.view.ViewGroup
       └── androidx.compose.ui.platform.AbstractComposeView
           └── androidx.compose.ui.platform.ComposeView

ComposeView extends androidx.compose.ui.platform.AbstractComposeView, which in turn extends android.view.ViewGroup, and ultimately, ViewGroup extends android.view.View.

Here’s an actual code snippet:

class ComposeView @JvmOverloads constructor(
    context: Context,
    attrs: AttributeSet? = null,
    defStyleAttr: Int = 0
) : AbstractComposeView(context, attrs, defStyleAttr) {

    private val content = mutableStateOf<(@Composable () -> Unit)?>(null)

    @Suppress("RedundantVisibilityModifier")
    protected override var shouldCreateCompositionOnAttachedToWindow: Boolean = false
        private set

    @Composable
    override fun Content() {
        content.value?.invoke()
    }

    override fun getAccessibilityClassName(): CharSequence {
        return javaClass.name
    }

    /**
     * Set the Jetpack Compose UI content for this view.
     * Initial composition will occur when the view becomes attached to a window or when
     * [createComposition] is called, whichever comes first.
     */
    fun setContent(content: @Composable () -> Unit) {
        shouldCreateCompositionOnAttachedToWindow = true
        this.content.value = content
        if (isAttachedToWindow) {
            createComposition()
        }
    }
}

Here,

Constructor:

• Uses @JvmOverloads to allow flexibility in calling the constructor with fewer parameters.
• Inherits from AbstractComposeView, which handles Jetpack Compose rendering in a View-based system.

State to Hold Content:

• content is a mutableStateOf variable that holds a nullable Composable lambda (@Composable () -> Unit).

Flag for Composition on Attachment:

• shouldCreateCompositionOnAttachedToWindow ensures that the composition is created when the view is attached to the window.
• By default, it’s false. It becomes true when setContent is called.

Composable Content Rendering:

• The Content() function is overridden from AbstractComposeView.
• If content is not null, it invokes the stored Composable lambda.

Accessibility:

• getAccessibilityClassName() returns the class name for accessibility purposes.

setContent Function:

• Accepts a Composable function and sets it to the content property.
• Sets shouldCreateCompositionOnAttachedToWindow to true.
• If the view is already attached to the window (isAttachedToWindow), it immediately calls createComposition() to render the content.

Adding ComposeView in XML

To add ComposeView in an XML layout, follow these steps:

Add Compose dependencies in your build.gradle:

// use latest versions
implementation "androidx.activity:activity-compose:1.7.2"
implementation "androidx.compose.ui:ui:1.6.0"
implementation "androidx.compose.material:material:1.6.0"

Add ComposeView in your XML layout:

<androidx.compose.ui.platform.ComposeView
    android:id="@+id/composeView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" />

Adding Composable to ComposeView in Activity/Fragment

In your Activity or Fragment, set the content for ComposeView.

val composeView = findViewById<ComposeView>(R.id.composeView)
composeView.setContent {
    MaterialTheme {
        Greeting("Compose in XML")
    }
}

@Composable
fun Greeting(name: String) {
    Text(text = "Hello, $name!", style = MaterialTheme.typography.h6)
}

Integrating ComposeView in a Fragment

If you are using ComposeView in a Fragment:

override fun onCreateView(
    inflater: LayoutInflater,
    container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    return ComposeView(requireContext()).apply {
        setContent {
            MaterialTheme {
                Greeting("Compose in Fragment")
            }
        }
    }
}

Why Use ComposeView?

• Incremental Migration: Migrate your app gradually without rewriting everything.
• Reuse Composables: Use powerful Composable functions in legacy projects.
• Flexibility: Combine both systems seamlessly.
• Modern UI Components: Bring Compose’s declarative UI and reactive state management to older architectures.

Key Differences: setContent vs setContentView

Best Practices for Using setContent

Keep setContent Clean and Simple:

• The lambda inside setContent should primarily call composable functions. Avoid complex logic inside the lambda to keep your code clean and readable.

Use Themes and Styling:

• Wrap your content in a theme (e.g., MaterialTheme) to ensure consistent styling across your app.

Separate Concerns:

• Structure your composables into separate functions and files based on their functionality. This improves readability and maintainability.

State Management:

• Use remember and mutableStateOf for local state management within composables. For shared state, consider using ViewModel and LiveData or StateFlow.

Common Pitfalls to Avoid

Blocking the UI Thread:

• Avoid long-running tasks or complex calculations inside setContent. Perform such tasks in a background thread using CoroutineScope.

Deeply Nested Composables:

• Keep composable functions small and focused to avoid deeply nested structures, which can affect performance and readability.

Ignoring State Changes:

• Ensure state changes trigger recomposition by using mutableStateOf or other state management solutions.

Conclusion

The setContent function is your entry point to building UI with Jetpack Compose. It replaces setContentView and opens the door to declarative, composable-based UI development. By understanding how setContent works and following best practices, you can create clean, maintainable, and dynamic user interfaces in your Android applications.

Jetpack Compose simplifies UI development, making it a pleasure to design responsive and interactive apps. Embrace setContent and the power of composables to elevate your Android development experience!